If a pulsar -- a dense star with strong gravitational
attraction -- is in a binary system, then it can pull in, or
accrete, material from its companion star. This influx of
material can eventually spin up the pulsar to the millisecond
range, rotating hundreds of revolutions per second.

Material accumulating on the pulsar surface can
sometimes ignite, causing thermonuclear flashes that emit
bursts of X-ray light. These thermonuclear flames spread
across the surface of the pulsar in a few seconds. The team
established that ďburst oscillationsĒ, a kind of flickering,
during these X-ray bursts provide a direct measure of the
pulsar's spin rate. This animation is a slow-motion depiction
of a thermonuclear flash or X-ray burst spreading across a
rotating pulsar. The pulsar would actually be rotating
hundreds of revolutions per second.

As the pulsar picks up speed through accretion, it
becomes distorted from a perfect sphere due to subtle changes
in the crust, depicted here by an equatorial bulge. Such
slight distortion is enough to produce gravitational waves.
Material flowing onto the pulsar surface from its companion
star tends to quicken the spin, but loss of energy released as
gravitational radiation tends to slow the spin due to the
principle of conservation of energy. This competition may
reach an equilibrium, setting a natural speed limit for
millisecond pulsars beyond which they cannot be spun
up.

Caption for Animation 5: SUPERNOVA ANIMATION: BIRTH OF A
PULSAR

A supernova is associated with the death of a star
about eight times as massive as the Sun or more. When such
stars deplete their nuclear fuel, they no longer have the
energy (in the form of radiation pressure outward) to support
their mass. Their cores implode, forming either a neutron star
(pulsar) or if there is enough mass, a black hole. The surface
layers of the star blast outward, forming the colorful
patterns typical of supernova remnants.

Gravitational radiation, ripples in the fabric of space
predicted by Albert Einstein, may serve as a cosmic traffic
enforcer, protecting reckless pulsars from spinning too fast
and blowing apart, according to a report published in the July
3 issue of Nature.

Pulsars, the fastest spinning stars in the Universe,
are the core remains of exploded stars, containing the mass of
our Sun compressed into a sphere about 10 miles across. Some
pulsars gain speed by pulling in gas from a neighboring star,
reaching spin rates of nearly one revolution per millisecond,
or almost 20 percent light speed. These "millisecond" pulsars
would fly apart if they gained much more speed.

Using NASA's Rossi X-ray Timing Explorer, scientists
have found a limit to how fast a pulsar spins and speculate
that the cause is gravitational radiation: The faster a pulsar
spins, the more gravitational radiation it might release, as
its exquisite spherical shape becomes slightly deformed. This
may restrain the pulsar's rotation and save it from
obliteration.

Animation
2

"Nature has set a speed limit for pulsar spins," said
Prof. Deepto Chakrabarty of the Massachusetts Institute of
Technology, lead author on the journal article. "Just like
cars speeding on a highway, the fastest-spinning pulsars could
technically go twice as fast, but something stops them before
they break apart. It may be gravitational radiation that
prevents pulsars from destroying themselves."

Gravitational waves, analogous to waves upon an ocean,
are ripples in four-dimensional spacetime. These exotic waves,
predicted by Einstein's theory of relativity, are produced by
massive objects in motion and have not yet been directly
detected.

Created in a star explosion, a pulsar is born spinning,
perhaps 30 times per second, and slows down over millions of
years. Yet if the dense pulsar, with its strong gravitational
potential, is in a binary system, it can pull in material from
its companion star. This influx can spin up the pulsar to the
millisecond range, rotating hundreds of times per second.

Animation 4

In some pulsars, the accumulating material on the
surface occasionally is consumed in a massive thermonuclear
explosion, emitting a burst of X-ray light lasting only a few
seconds. In this fury lies a brief opportunity to measure the
spin of otherwise faint pulsars. Scientists report in Nature
that a type of flickering found in these X-ray bursts, called
"burst oscillations," serves as a direct measure of the
pulsar's spin rate. Studying the burst oscillations from 11
pulsars, they found none spinning faster than 619 times per
second.

The Rossi Explorer is capable of detecting pulsars
spinning as fast as 4,000 times per second. Pulsar break-up is
predicted to occur at 1,000 to 3,000 revolutions per second.
Yet scientists have found none that fast. >From statistical
analysis of 11 pulsars, they concluded that the maximum speed
seen in nature must be below 760 revolutions per second.

Animation 5

This observation supports the theory of a feedback
mechanism involving gravitational radiation limiting pulsar
speeds, proposed by Prof. Lars Bildsten of the University of
California, Santa Barbara. As the pulsar picks up speed
through accretion, any slight distortion in the star's dense,
half-mile-thick crust of crystalline metal will allow the
pulsar to radiate gravitational waves. (Envision a spinning,
oblong rugby ball in water, which would cause more ripples
than a spinning, spherical basketball.) An equilibrium
rotation rate is eventually reached where the angular momentum
shed by emitting gravitational radiation matches the angular
momentum being added to the pulsar by its companion star.

Bildsten said that accreting millisecond pulsars could
eventually be studied in greater detail in an entirely new
way, through the direct detection of their gravitational
radiation. LIGO, the Laser Interferometer Gravitational-Wave
Observatory now in operation in Hanford, Washington, and in
Livingston, Louisiana, will eventually be tunable to the
frequency at which millisecond pulsars are expected to emit
gravitational waves.

"The waves are subtle, altering spacetime and the
distance between objects as far apart as the Earth and the
Moon by much less than the width of an atom," said Prof. Barry
Barish of the California Institute of Technology, the LIGO
director. "As such, gravitational radiation has not been
directly detected yet. We hope to change that soon."

Stills from the animations:

Animation Still 1

Animation Still 2

Downloadable high resolution copies can be found on the
navigation bar to the right.